1. Field of the Invention
The present invention relates to a filtering apparatus for filtering liquid such as water. More particularly, the invention relates to a technique for achieving efficient filtration wherein the momentum of the liquid discharged upward is effectively restricted within a small space, thereby to allow uniform infiltration of the liquid over a surface of a filter medium inside a relatively small filtering tower.
2. Description of the Related Art
A conventional filtering apparatus shown in FIG. 9 includes, within a filtering tower 1 having a lid, a filtering section (r) comprising a plurality of kinds of filter media, a liquid collecting section 7 disposed downwardly of the filtering section (r), and a liquid feeding section (e) disposed upwardly of the filtering section (r). The liquid feeding section (e) includes a liquid feeding pipe 5 inserted into the filtering tower 1 from the outside thereof and having an orifice 5a oriented upward and formed at the leading end thereof, and a diffuser (A) for acting on the liquid being discharged upward from the orifice 5a. 
The diffuser A is a tapered construction whose diameter increases upward. In other words, this diffuser comprises a funnel-like member for allowing the liquid to be filtered to be discharged while being diffused toward the outer peripheral side over a wide range from the upward direction to obliquely upward directions. With this, a portion of the liquid discharged from the orifice 5a is caused to run along a ceiling face 1b of the filtering tower 1 while another portion thereof is caused to fall before reaching the ceiling face 1b. Consequently, the filtering liquid is intended to fall uniformly onto over substantially entire top surface of a filter medium layer 4.
However, it has been shown that when the filtering apparatus having the above-described conventional construction is actually put into use, as indicated by arrows in FIG. 9, most of the filtering liquid discharged from the diffuser A flows down along the ceiling face 1b and an inner peripheral surface of a lateral face 1a, that is, the liquid flows down in a concentrated manner on the outer peripheral side inside the cylindrical filtering tower 1. And, it has been confirmed that by the momentum of the liquid concentrated on the outer peripheral side of the filtering tower 1, the filter medium layer 4 is disturbed and stirred up and displaced toward the vertical axis of the filtering tower 1, so that as shown in FIG. 9 the upper surface of the filter medium layer 4 is formed into a mountain-like shape with the inner radius side thereof being raised and its outer radius side thereof being depressed. That is to say, in the actual use, the liquid is concentrated on the outer peripheral side and may not be diffused sufficiently over the entire top surface of the filter medium layer 4. As a result, the flatness of the top surface of the filter medium layer 4 tends to be impaired, thus tending to invite insufficient filtering performance of the filter medium layer 4. In this respect, there has been room for improvement.
The occurrence of the above-described problem seems to be attributable to a background situation as follows. Namely, as the filter media employed by the conventional filtering apparatus, medium consisting mainly of sand having a relative large specific gravity was often employed previously. Hence, when the liquid is discharged at a standard discharge speed (2 m/sec approximately), the filtering operation was possible with maintaining the flatness of the top surface of the filter medium layer 4. In recent years, however, in order to allow effective back washing of the filter medium layer 4 with a limited amount of water, media comprising special ceramic, activated carbon or the like having a relatively small specific gravity have often been employed. With such decrease in the specific gravity of the material forming the filter medium layer 4, the above-described phenomenon of stirring-up of the filter media and the resultant impairment of the flatness of the top surface of the filter medium layer 4 tends to occur more likely (the term “back washing” refers to an operation or process in which in the event of reduction in the filtering ability due to e.g. accumulation of solid content in the filtering liquid on the filter medium layer, the water is discharged from the liquid collecting section 7 provided at the lower portion of the filtering tower and adapted normally for collecting the filtered water or the like, thereby to release the solids from the filter medium layer 4, so that the released solids may be discharged together with the back washing water through the diffuser A operable normally for discharging the liquid to be filtered).
Therefore, with such reduction in the weight of the filter medium, even with the discharging speed of water remaining the same as before, the surface of the filter medium tends to be disturbed to impair the flatness of the top face of the filter media. Hence, concentration of the filtering liquid occurs at the bottomed or lowered areas in the filter media surface, thus resulting in deterioration of filtering precision as well as reduction in the effective volume of the filter media 4 actually effective for entrapment of solids or the like contained in the filtering liquid. Consequently, the filtering operation period will be reduced disadvantageously and the filtering efficiency will also suffer.
In order to achieve uniformity in the falling movement of the filtering liquid for solving the above-described inconvenience, it is conceivable to reduce the discharging speed of water or conversely to increase the capacity of the filtering tower. With the former method, however, the filtering ability per unit time period, namely, the filtering efficiency, will be reduced. With the latter method, the space required for installing the filtering tower and the cost of the tower will increase disadvantageously, malting the existing system unusable. Hence, in reality, neither method have been able to provide an effective solution.
An object of the present invention is to provide an apparatus which allows use of the existing filtering tower without its enlargement and which allows uniform falling of the filtering liquid over the substantially entire top surface of the filter medium layer 4, thus maling it readily possible to maintain the flatness of the top surface of the filter medium layer 4 even when light-weight medium is employed, thereby to allow the filter medium layer 4 as well as the entire filtering apparatus to achieve their desired filtering performance.